1. Field of the Invention
The present invention relates to an ignition system for a vehicle internal combustion engine, and, particularly, a multi-spark type ignition system that repeatedly ignites fuel each time it is supplied to each cylinder of an internal combustion engine.
2. Description of the Related Art
U.S. Pat. Nos. 4,892,080 and 5,056,496 disclose multi-spark type ignition systems in which as many ignition coils as cylinders of an internal combustion engine are employed. Such a system also includes an energy storage coil, a energy storage capacitor and switching power transistors for intermittently controlling supply of electric energy. One of the switching transistors switches on or off current flowing through the energy storage coil according to ignition timing signals. Because the energy storage coil supplies energy to all the ignition coils, a considerably large amount of electric current flows through the energy storage coil. Therefore, the resistance of the energy storage coil is required to be very low in order to prevent the temperature of the energy storage coil from excessively rising due to Joule heat.
However, if the power transistor is kept turn on due to some failure, the power transistor may be heated more as the resistance of the energy storage coil becomes smaller because the current flowing through the power transistor can not be suitably limited. Therefore, it was necessary to limit the electric current to prevent temperature rise the power transistor while maintaining the coil stored energy by increasing the number of turns thereof and to increase the size of the energy storage coil in order to increase heat radiation thereof.